The nature of and basic approaches to cancer treatment are constantly changing. At present, adjuvant chemotherapy routinely follows local treatment of cancers. Clinical protocols are now exploring genetic therapies, manipulations of the immune system, stimulation of normal hematopoietic elements, induction of differentiation in tumor tissues, and inhibition of angiogenesis. Research in these new areas has led to applications for nonmalignant disease.
At the same time, the new clinical protocols have a narrow therapeutic index as well as a great potential for causing harmful side effects. A thorough understanding of the pharmacology, drug interactions, and clinical pharmacokinetics is essential for safe and effective use in human beings.
The therapy of viral infection is in its infancy. Bacterial infection is typically treated with agents, such as antibiotics, which take advantage of the differences in metabolism between the infecting organism and its host. However, viruses largely employed the host's own enzymes to effect the replication, and thus leave few opportunities for pharmacological intervention. By employing strong regulatory elements, the virus obtains transcription and translation of its own genes at the expense of host genes.
In mammals, viral infection is combated naturally by cytotoxic T-lymphocytes, which recognize viral proteins when expressed on the surface of host cells, and lyse the infected cells. Destruction of the infected cell prevents the further replication of the virus. Other defenses include the expression of interferon, which inhibits protein synthesis and viral budding, and expression of antibodies, which remove free viral particles from body fluids. However, induction of these natural mechanisms require exposure of the viral proteins to the immune system. Many viruses exhibit a dormant or latent phase, during which little or no protein synthesis is conducted. The viral infection is essentially invisible to the immune system during such phases.
Retroviruses carry the infectious form of their genome in the form of a strand of RNA. Upon infection, the RNA genome is reverse-transcribed into DNA, and is typically then integrated into the host's chromosomal DNA at a random site. On occasion integration occurs at a site which truncates a gene encoding an essential cellular receptor or growth factor, or which places such a gene under control of the strong viral cis-acting regulatory element, which may result in transformation of the cell into a malignant state.
Viruses may also be oncogenic due to the action of their trans-acting regulatory factors on host cell regulatory sequences. In fact, oncogenesis was the characteristic which led to the discovery of the first known retroviruses to infect humans. HTLV-I and HTLV-II (human T-lymphotrophic viruses I and II) were identified in the blood cells of patients suffering from adult T-cell leukemia (ATL), and are believed to induce neoplastic transformation by the action of their transactivating factors on lymphocyte promoter regions. Two additional retroviruses have been found to infect humans. These viruses, HIV-I and HIV-II, are the etiological agents AIDS.
Current therapy for HIV infection includes new drugs called protease inhibitors. These drugs can dramatically reduce HIV levels in the blood when taken with other antiviral compounds such as AZT. At the same time, natural weapons in the immune system's defenses polypeptide molecules called chemokines, have been unveiled as potent foes of HIV.
Antisense oligodeoxynucleotides have been proposed as a major class of new pharmaceuticals. In general, antisense refers to the use of small, synthetic oligonucleotides resembling single-stranded DNA, to inhibit gene expression. Gene expression is inhibited through hybridization to coding (sense) sequences in a specific messenger RNA (mRNA) target by Watson-Crick base pairing in which adenosine and thymidine or guanosine and cytidine interact through hydrogen bonding.
Following the simple base-pairing rules which govern the interaction between the antisense oligodeoxynucleotides and the cellular RNA, allow the design of molecules to target any gene of a known sequence. A major advantage of this strategy is the potential specificity of action. In principal, an antisense molecule can be designed to target any single gene within the entire human genome, potentially creating specific therapeutics for any disease in which the causative gene is known. As a result, there have been numerous applications of antisense oligodeoxynucleotide (ODN) activity for potential antiviral and anticancer applications.
Antisense ODNs offer the potential to block the expression of specific genes within cells. Despite numerous reports of apparent antisense inhibition of gene expression in cultured cells, only in a few cases has specific inhibition been rigorously demonstrated. In many studies, specificity has been inferred from the biological effects of antisense as compared to control ODNs, without measuring levels of target RNA or proteins to evaluate specificity. Unintended side-effects of antisense technology could potentially occur through a number of mechanisms.
The potential of oligonucleotides as modulators of gene expression is currently under intense investigation. Most of the efforts are focused on inhibiting the expression of targeted genes such as oncogenes or viral genes. The oligonucleotides are directed either against RNA (antisense oligonucleotides) or against DNA where they form triplex structures inhibiting transcription by RNA polymerase II. To achieve a desired effect, the oligonucleotides must promote a decay of the preexisting, undesirable protein by effectively preventing its formation de novo.
There is therefore a need for the development of new antisense methods that are more potent, reliable and specific than those used in previous studies.